Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis.
Ample evidence points to central role of NF-kB in such pathologic
responses, yet the precise mechanisms underpinning specificity of these
responses remain unclear. We propose that motifs of the scaffold protein
IKKg/NEMO partly facilitate such functions. As proof-of-principle, we used
site-specific mutagenesis to examine the role of NEMO in mediating
RANKL-induced signaling in bone marrow macrophages, known as osteoclast
precursors. We identified lysine (K)270 as a target regulating RANKL
signaling as K270A substitution results in exuberant osteoclastogenesis in
vitro and inflammatory osteolysis in vivo. Mechanistically, we discovered
that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates
inflammatory burden. Specifically, K270A directly or indirectly hinders
binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets
the modified proteins to autophagy-mediated lysosomal degradation. Taken
together, our findings suggest that NEMO serves as a toolkit to fine-tune
specific signals in physiologic and pathologic conditions.